The present invention relates to an installation designed for reabsorbing the mechanical tensions induced in a tube by an expansion operation. Such an installation is particularly suitable for the detensioning of the ends of heat exchanger tubes and more especially steam generators, particularly when these equipments are used in a nuclear reactor. However, it is not limited to such an application.
In heat exchangers, the heat exchange between the two fluids generally takes place through a group of long tubes, in which one of the fluids circulate, whereas the other fluid circulates around the tubes. At their ends, the tubes are tightly fixed to a thick plate, called a "tube plate". Usually this tight fixing is obtained on the one hand by welding the tube end to the outer face of the plate and on the other by the expansion of the tube over at least part of the thickness of the tube plate, said part generally being located in the vicinity of the inner face of the plate.
The expansion of the tubes in the tube plate leads to mechanical stresses in the tubes generally located in the vicinity of the inner skin of said tubes. In certain cases, these mechanical stresses have no effect on the operation of the exchanger and are consequently admissible.
However, when the fluids circulating in the exchanger are corrosive, as is more particularly the case in nuclear reactors, these stresses lead to the production of microcracks, which can sometimes completely traverse the tube, which consequently becomes unusable and has to be sealed. This makes it necessary to carry out checking operations, followed by operations for sealing the tubes on a relatively frequent basis. Moreover, the sealing of a large number of tubes of a heat exchanger leads to a reduction in its efficiency by the same proportion.
It is therefore desirable to have an installation making it possible to reabsorb the stresses induced in tubes during expansion.
A first known method for carrying out the detensioning of expanded tubes is shotpeening developed by FRAMATOME. This method consists of introducing into the expanded part of the tube an injection head by which Inconel shot is loosely blasted against the inner wall of the tube by means of compressed air. The resulting peening reabsorbs the internal skin stresses.
This shotpeening method is particularly effective when the conditions under which it is applied are satisfactory. However, the maintaining of this effectiveness makes it necessary to very strictly control the temperature and inject the shot in the absence of any humidity. Moreover, when this method is used on a heat exchanger equipping an operating nuclear reactor, the shot used for the detensioning of the tubes are contaminated. Bearing in mind the number of tubes generally equipping a heat exchanger, a large volume of waste requiring subsequent treatment is consequently produced. The cost of this method is therefore greatly increased.
Another known method for detensioning expanded tubes is rotopeening developed by WESTINGHOUSE. According to this method, described in FR-A-No. 2 511913, the detensioning of the expanded tubes is carried out by successively introducing into each of the tubes a tool ensuring the peening of the inner wall of the tube. This tool is formed from flexible plates (generally called brushes) mounted on a rotary shaft and carrying tungsten carbide shot at the ends thereof. The rotary shaft is eccentrically supported by a cylindrical cage centered by bearings within the tube to be treated. At each of the brushes, the cage has an open window in its part closest to the shaft. Thus, the rotation of the latter brings about the peening of the tube parts facing said windows by the shot carried by the brushes. In order that peening takes place over the entire tube circumference, the cage is also relatively slowly rotated about its own axis. Finally, the covering of the entire length of the expanded zone is obtained by imparting a reciprocating alternating movement to the cage parallel to its axis.
Compared with shotpeening, this rotopeening method has the essential advantage of not producing waste requiring special treatment and is also neither sensitive to temperature, nor humidity.
However, bearing in mind the superimposing of a rotary movement and an alternating reciprocating axial movement of the cage for circumferentially and longitudinally covering the expanded zone, the distribution of the impact of the shot on the inner wall of the tube is not uniform. This either makes the method partly ineffective, or leads to an excessively long operation to be satisfactory from the industrial standpoint.
The present invention constitutes an improvement to the rotopeening method, whose main objective is to ensure a regular distribution of the impact of the shot on the inner wall of the tubes, so that the minimum number of impacts necessary for obtaining a complete treatment of the surface is obtained in a sufficiently short time to be acceptable from the industrial standpoint.